1. Field of the Invention
This application relates to operational amplifier methods and systems and more particularly to methods and systems for frequency compensation of class AB operational amplifiers (i.e., "opamps") with fully-differential input stages.
2. Description of Related Art
Some prevalent operational amplifier designs require frequency compensation to meet predetermined operational standards. In particular, class AB operational amplifiers having a single ended output and fully-differential input stages require frequency compensation to operate at an acceptable level. Single-ended class AB operational amplifiers have fully-differential input stages in some instances. This is because the non-linearity of the amplifier is dominated by the non-linearity of the input stage in many cases. A fully differential input stage is a balanced structure which is very linear. The non-linearity of the second stage is divided by the gain of the input stage.
A common way of compensating two-stage operational amplifiers is to connect a pole-splitting capacitor between a first stage high-impedance node and a second stage high-impedance node. The two stages are inverting with respect to each other. This causes one of the poles to move to a low frequency level, with the pole becoming dominant, while the other pole moves to high frequency. According to Paul Gray and Robert Meyer in Analysis and Design of Analog Integrated Circuits (Wiley), if the new high frequency pole is above the crossover frequency of the operational amplifier, the stability of the operational amplifier is not detrimentally affected.
Operational amplifiers are configured as line drivers in digital subscriber lines (DSL) and as preamplifiers in sensing systems having an input sensor connected to a preamplifier which in turn drives an analog to digital converter. ADSL and DSL Technologies by Walter Goralski (McGraw-Hill 1998) describes DSL systems in which such operational amplifiers can be used.
A major technical problem in the design of single-ended output and fully-differential input operational amplifiers is degraded performance due to lack of balanced frequency compensation. Accordingly, circuit designers avoid using such operational amplifiers because of their inadequate performance characteristics. Accordingly, it is desired to develop more refined operational amplifier designs that will enable class AB amplifiers to operate at enhanced levels without suffering from degraded performance.